Switched-mode power supplies (SMPS) are widely used for supplying power to electronic devices (loads) such as computers, television sets, lighting systems and other electronic appliances. As with other types of power supplies, an SMPS converts power from a power source into a form more appropriate for a load. In typical applications, the source power is obtained from an alternating current (AC) mains voltage with a frequency of 50 Hz to 60 Hz and root mean square (RMS) voltage levels of 90V to 240V. The SMPS converts this source power into a lower-voltage direct current (DC) power supplied to the load.
SMPS have several advantages over other power supply types. SMPS are often significantly more efficient than other power supply types as they waste less energy in ohmic losses, e.g., as done by power supplies relying on linear regulation. Because wasted energy must typically be dissipated as heat, SMPS have reduced heat dissipation requirements meaning fewer and/or smaller heat sinks are required. Furthermore, some SMPS configurations require no or a smaller transformer than other power supply types. For these reasons, SMPS are often smaller and less expensive than other power supply types.
While there are many configurations for SMPS, all of them switch (enable and disable), at some level, power that is supplied to an output load. An SMPS controller governs this switching by varying parameters such as the frequency and/or duty cycle of a pulse-width-modulated (PWM) signal that is used to drive one or more switch devices, e.g., transistors, such that the output power supplied to a load meets the load's power requirements in terms of voltage and current.
The SMPS controller is, itself, an electronic device that must be powered. Power must be supplied to the SMPS controller during the start-up of the SMPS and for the continuing (steady-state) operation of the SMPS. Some prior solutions for supplying power to the SMPS controller have relied upon a separate power supply that is external to the SMPS. Other solutions have tapped an auxiliary winding of a transformer within the SMPS to supply power to the SMPS controller. Use of an external power supply has the disadvantage that it requires extra circuitry thereby increasing the size and cost of the SMPS system. Use of the auxiliary winding of a transformer requires that the SMPS include a transformer, which may not be desirable in some SMPS designs, e.g., buck converters without a transformer. Even for SMPS designs that incorporate a transformer, it may be preferred to not require the inclusion of an auxiliary winding (having an associated size and cost) within the transformer.
U.S. patent application Ser. No. 14/974,668 discloses techniques for providing power to an SMPS controller. These techniques use a normally-on switch device that is controlled by a bang-bang controller. While these techniques address many of the issues discussed above, the losses associated with the normally-on device, which is typically connected in series across a high-voltage input power source, lead to inefficiencies that would preferably be avoided. Additionally, the bang-bang controller may require additional circuitry and/or complexity, which would also preferably be avoided or, at least, minimized.
Circuits and methods for supplying power to an SMPS controller are desired. Such circuits and methods should require minimal circuit components beyond what is required for the SMPS, and should be capable of efficiently supplying power to an SMPS controller during start-up and steady-state operation of the SMPS.